Multi-level signaling

ABSTRACT

Apparatus are disclosed, such as those involving a transmitter circuit that is configured to generate multi-level signals based on a plurality of data digits. One such transmitter circuit includes a signal output and an encoder configured to provide control signals based at least partially on the plurality of data digits. The transmitter circuit also includes a first set of switches configured to receive one or more of the control signals, and to selectively conduct a first or second voltage reference to the signal output. The transmitter circuit further includes first and second voltage drop circuits that provide third and fourth voltage references, respectively. The third and fourth voltage references have voltage levels between those of the first and second voltage references. The transmitter circuit also includes a second set of switches configured to receive one or more of the control signals, and selectively conduct the third or fourth voltage reference to the signal output.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No. 13/227,319, filed Sep. 7, 2011, titled “LOW POWER MULTI-LEVEL SIGNALING,” which is a continuation of U.S. patent application Ser. No. 12/053,265, filed Mar. 21, 2008, titled “MULTI-LEVEL SIGNALING FOR LOW POWER, SHORT CHANNEL APPLICATIONS,” now U.S. Pat. No. 8,026,740, the disclosures of each which are hereby incorporated by reference in their entireties herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Embodiments of the invention relate to electronic data transmission, and more particularly, in one or more embodiments, to multi-level signaling.

2. Description of the Related Art

In electronic data transmission, signals representing electronic data are transmitted from a transmitting component to a receiving component. The electronic data typically includes data symbols such as binary digits (which are often referred to as bits), i.e., 0's and 1's. In many applications, signals having two different voltage levels representing data symbols such as 0's and 1's have been widely used for such data transmission.

In certain applications, signals having more than two levels have been used to transmit electronic data. Such a signaling scheme is generally referred to as “multi-level signaling.” In a multi-level signaling scheme, the data rate can be increased without increasing the clock frequency or the number of channels. In such a scheme, transmitters generate multi-level signals, and receivers detect them, allowing multiple (k) bits to be transmitted or received as one of 2^(k) possible voltages at each clock edge or at least once per clock cycle. A multi-level signaling scheme allows two or more bits of data to be simultaneously transmitted by multiple signal levels, thereby increasing the data throughput for a given operating frequency.

In certain instances, the term “multi-level signaling” is also referred to as multi-level pulse amplitude modulation or M-PAM signaling. In such a case, the number of signal levels is indicated by a number before the acronym “PAM.” For example, a PAM signaling scheme using four signal levels is represented by 4 PAM. Similarly, a PAM signaling scheme using eight signal levels is represented by 8 PAM.

FIG. 1 is a graph illustrating a conventional multi-level signaling scheme using four voltage levels, i.e., 4 PAM. The four signal levels represent two-bits b₀ b₁. The highest voltage level over a high reference voltage V_(REFH) represents the bits “11.” The second highest voltage level between the high reference voltage V_(REFH) and a middle reference voltage V_(REFM) (which is lower than V_(REFH)) represents the bits “10.” The third highest voltage level between the middle reference voltage and a low reference voltage V_(REFL) (which is lower than V_(REFM)) represents the bits “01.” The lowest voltage level lower than V_(REFL) represents the bits “00.” The two bits are transmitted as a single multi-level symbol at every clock edge by transferring an appropriate one of the four voltage levels. Therefore, the data rate of the signaling scheme just described is twice that of a 2-PAM system.

Referring to FIG. 2, a conventional transmitter circuit 200 for generating voltage levels under a 4 PAM signaling scheme will now be described. To provide the voltage levels to transmit a 4-PAM symbol, the transmitter circuit 200 sinks a predetermined amount of current for that symbol. In particular, each symbol is associated with a distinct amount of current.

The transmitter circuit 200 includes a voltage source V_(DD), a resistor R, a first transistor TR1, a second transistor TR2, a first current source CS1, a second current source CS2, a first node N1, and a signal output V_(OUT). The resistor R is connected between the voltage source V_(DD) and the first node N1. The first node N1 is electrically connected to the signal output V_(OUT). Each of the first and second transistors TR1, TR2 is connected to the first node N1 at its source/drain. The drain/source of the first transistor TR1 is connected to the first current source CR1 which provides a current of 2I. The drain/source of the second transistor TR2 is connected to the second current source CR2 which provides a current of I.

To transmit the bits “11,” the transmitter circuit 200 sinks no current by turning off both of the first and second transistors TR1, TR2, and the signal output \T_(OUT) is pulled up to V_(DD). To transmit the bits “10,” the transmitter circuit 200 sinks an amount of current I by turning on the second transistor TR2 only, thereby providing V_(DD)−RI at the signal output \T_(OUT). To transmit the bits “01,” the transmitter circuit 200 sinks an amount of current 21 by turning the first transistor TR1 only, thereby providing V_(DD)−2RI at the signal output V_(OUT). To transmit the bits “00,” the transmitter circuit 200 sinks an amount of current 31 by turning on both of the first and second transistors TR1, TR2, thereby providing V_(DD)−3RI at the signal output V_(OUT).

In the transmitter described above, when generating three out of the four signal levels (e.g., those representing “00”, “01”, and “10”), power is dissipated because there is a current flow through the resistor R in the circuit. In other words, three out of four signal levels consume static power. It should be understood that the translation of the two sequential binary bits into multiple voltage levels need not follow exactly as it has been described thus far. For example, the bits “00” could be encoded as the highest voltage level, rather than as the lowest level, as has thus far been illustrated. Other alternative exist as well, including the well known gray-coding which would order the levels as follows: “00”, “01”, “11”, and “10.”

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments will be better understood from the Detailed Description of Embodiments and from the appended drawings, which are meant to illustrate and not to limit the embodiments, and wherein:

FIG. 1 is a graph illustrating a conventional multi-level signaling scheme;

FIG. 2 is a circuit diagram of a conventional transmitter circuit for generating a four-level voltage signal;

FIG. 3A is a block diagram of a transmitter circuit for generating a four-level signal according to one embodiment;

FIG. 3B is a block diagram of a transmitter circuit for generating a four-level signal according to another embodiment;

FIG. 4 is a circuit diagram of a transmitter circuit for generating a four-level signal according to one embodiment;

FIG. 5 is a graph illustrating a multi-level signaling scheme using the transmitter circuit of FIG. 4 according to one embodiment;

FIG. 6 is a circuit diagram of a transmitter circuit for generating a four-level signal according to another embodiment;

FIG. 7 is a circuit diagram of a transmitter circuit for generating a four-level signal according to yet another embodiment;

FIG. 8 is a circuit diagram of a transmitter circuit for generating a four-level signal according to yet another embodiment;

FIG. 9 is a circuit diagram of a transmitter circuit for generating an eight-level signal according to yet another embodiment;

FIG. 10 is a block diagram of a transmitter circuit for generating a four-level signal according to another embodiment; and

FIG. 11 is a circuit diagram of a transmitter circuit for generating a four-level signal according to another embodiment.

DETAILED DESCRIPTION OF EMBODIMENTS

In one embodiment, a transmitter circuit for generating a multi-level signal includes a push-pull circuit. The push-pull circuit may include a plurality of voltage drop circuits and a plurality of switches to pull up or pull down its signal output level to one of multiple voltage levels.

Referring to FIG. 3A, one embodiment of a transmitter circuit for multi-level signaling will now be described. The illustrated transmitter circuit 300A is configured to generate a four level voltage signal, i.e., a 4-PAM signal, based on two binary digits b₀ and b₁. The transmitter circuit 300 includes a first set of switches 310, a second set of switches 320, a first voltage drop circuit 330, a second voltage drop circuit 340, an encoder 350, a data input 360, and a signal output 370.

The first voltage drop circuit 330, the first set of switches 310, and the second voltage drop circuit 340 are connected in order between a first voltage reference V_(R1) and a second voltage reference V_(R2). The first set of switches 310 may include a first pull-up transistor 310 a and a first pull-down transistor 310 b. A first node N1 between the first pull-up transistor 310 a and the first pull-down transistor 310 b is electrically connected to the signal output 370. The first voltage drop circuit 330 is connected between the first voltage reference V_(R1) and the first pull-up transistor 310 a. The second voltage drop circuit 340 is connected between the second voltage reference V_(R2) and the first pull-down transistor 310 b. The first voltage reference V_(R1) may be provided by a voltage source, e.g., V_(DD). The second voltage reference V_(R2) is lower in potential than the first voltage reference V_(R1), and in one embodiment is ground.

The second set of switches 320 is connected between the first and second voltage references V_(R1), V_(R2). The second set of switches 320 may include a second pull-up transistor 320 a and a second pull-down transistor 320 b. A second node N2 between the second pull-up transistor 320 a and the second pull-down transistor 320 b is electrically connected to the signal output 370.

The encoder 350 receives electronic data from the data input 360, and converts it into control signals S1-S3 for selectively switching on or off the components of the first and second sets of switches 310, 320, depending on the electronic data. In the illustrated embodiment, the encoder 350 generates the control signals S1-S3 in response to two bits of electronic data at a time. The details of the control signals S1-S3 will be described below with reference to FIG. 4. In addition, further details of the voltage drop circuits and switches will be described later in connection with FIG. 4.

During operation, the transmitter circuit 300A may generate a signal having one of four voltage levels V1, V2, V3, and V4 (V1>V2>V3>V4). In one embodiment, each of V1, V2, V3, and V4 may each represent two-bits: 11, 10, 01 and 00, respectively. For example, when the bits 11 are provided to the data input 360 (e.g., two signals are received at the data input, each being at one of two possible levels), the encoder 350 generates control signals such that the second set of switches 320 pulls up the signal output 370 to the first reference voltage V_(R1), which corresponds to V1. When the bits 00 are provided to the data input 360, the encoder 350 generates control signals such that the second set of switches 320 pulls down the signal output 370 to the second reference voltage V_(R2), which corresponds to V4.

In an alternative embodiment, the first set of switches 310 are also configured to pull up or down with the second set of switches 320 to aid in reaching the full voltage levels. This configuration allows the second set of switches 320 to be smaller, thereby reducing the output capacitance of the circuit, which is important at high speeds.

When the bits 10 or 01 are provided to the data input 360, the encoder 350 generates control signals such that the second set of switches 320 electrically disconnects the signal output 370 from the first and second reference voltages V_(R1), V_(R2). When the bits 10 are provided to the data input 360, the first set of switches 310 electrically connects the first voltage drop circuit 330 to the signal output 370, and electrically disconnects the second voltage drop circuit 340 from the signal output 370. This configuration provides a voltage level of the first reference voltage V_(R1) less a first voltage difference V_(D1) across the first voltage drop circuit 330 (V_(R1)−V_(D1)) at the signal output 370.

On the other hand, when the bits 01 are provided to the data input 360, the first set of switches 310 electrically connects the second voltage drop circuit 340 to the signal output 370, and electrically disconnects the first voltage drop circuit 330 from the signal output 370. This configuration provides a voltage level of the second reference voltage V_(R2) plus a second voltage difference V_(D2) across the second voltage drop circuit 340 (V_(R2)+V_(D2)) at the signal output 370. In the illustrated embodiment, V_(R1)−V_(D1) is greater than V_(R2)+V_(D2). In some embodiments, V_(D1) is the same as V_(D2). The aforementioned signal levels representing the bits 11, 10, 01, and 00 are summarized in Table 1 below.

TABLE 1 b₀ b1 V_(OUT) 1 1 V_(R1) 1 0 V_(R1) − V_(D1) 0 1 V_(R2) + V_(D1) 0 0 V_(R2)

Referring to FIG. 3B, another embodiment of a transmitter circuit for multi-level signaling will now be described. The configuration of the illustrated transmitter circuit 300B is the same as that of the transmitter circuit 300A of FIG. 3A except that the encoder 350 provides four control signals S1-S4, instead of three control signals S1-S3.

In this embodiment, a first control signal S1 is provided to the first pull-up transistor 310 a while a second control signal S2 is provided to the first pull-down transistor 310 b. A third control signal S3 and a fourth control signal S4 are provided to the second pull-up and pull-down transistors 320 a, 320 b, respectively. A skilled artisan will appreciate that the encoder 350 may generate any suitable combinations of control signals for performing the same function as the transmitter circuit 300A of FIG. 3A.

Referring to FIG. 4, a circuit diagram of a transmitter circuit for multi-level signaling according to one embodiment will now be described. The illustrated transmitter circuit 400 is configured to generate a four level voltage signal, i.e., a 4-PAM signal, based on two binary digits b₀ and b₁. The transmitter circuit 400 includes a first set of switches 410, a second set of switches 420, a first voltage drop circuit 430, and a second voltage drop circuit 440, an encoder 450, a data input 460 and a signal output 470. The electrical connection among the foregoing components can be described as above with respect to the circuit 300A of FIG. 3A.

The first set of switches 410 may include a first transistor TR1 of a first type, and a second transistor TR2 of a second type opposite from the first type. In the illustrated embodiment, the first transistor TR1 is a p-type MOS transistor, and the second transistor TR2 is an n-type MOS transistor. In another embodiment, the first transistor TR1 may be an n-type MOS transistor, and the second transistor TR2 may be a p-type MOS transistor. In other embodiments, the first and second transistors TR1, TR2 may be replaced with bipolar transistors (see BTR1 and BTR2 in FIG. 11) or other field effect transistors of opposite types.

Each of the first and second transistors TR1, TR2 has a source/drain, a gate, and a drain/source. The drain/source of the first transistor TR1 and the drain/source of the second transistor TR2 are electrically connected to a first node N1, which is electrically connected to the signal output 470.

The second set of switches 412 may include a third transistor TR3 of the first type, and a fourth transistor TR4 of the second type opposite from the first type. In the illustrated embodiment, the third transistor TR3 is a p-type MOS transistor, and the fourth transistor TR4 is an n-type MOS transistor. In other embodiments, the third and fourth transistors TR3, TR4 may be replaced with bipolar transistors (see BTR3 and BTR4 in FIG. 11) or other field effect transistors of opposite types.

Each of the third and fourth transistors TR3, TR4 has a source/drain, a gate, and a drain/source. The drain/source of the third transistor TR3 and the drain/source of the fourth transistor TR4 are electrically connected to a second node N2, which is electrically connected to the signal output 470 and the first node N1. The drain/source of the third transistor TR3 is electrically connected to a voltage source V_(DD). The source/drain of the fourth transistor TR4 is electrically connected to ground.

The first voltage drop circuit 430 and the second voltage drop circuit 440 may include a fifth transistor TR5 and a sixth transistor TR6, respectively. In the illustrated embodiment, the fifth transistor TR5 is a p-type MOS transistor and the sixth transistor TR6 is an n-type MOS transistor. In another embodiment, the fifth transistor TR5 may be an n-type MOS transistor, and the sixth transistor TR6 may be a p-type MOS transistor.

Each of the fifth and sixth transistors TR5, TR6 has a source/drain, a gate, and a drain/source. The source/drain of the fifth transistor TR5 is electrically connected to the voltage source V_(DD), and the drain/source of the fifth transistor TR5 is electrically connected to the source/drain of the first transistor TR1. The drain/source of the sixth transistor TR6 is electrically connected to the source/drain of the second transistor TR2, and the source/drain of the sixth transistor TR6 is electrically connected to ground.

The gates of the fifth and sixth transistors TR5, TR6 are electrically connected to the first node N1. Thus, the fifth and sixth transistors TR5, TR6 form diode-connected transistors, functioning as diodes during the operation of the transmitter circuit 400. In other embodiments, each of the first and second voltage drop circuits 430, 440 may include two or more diode-connected transistors connected in series.

The encoder 450 includes a first inverter IV1, a second inverter IV2, a NAND gate 451, and a NOR gate 452. The first and second inverters IV1, IV2 receive a first bit b₀ of two-bit electronic data b₀ b₁, and invert the first bit. The first and second inverters IV1, IV2 provide the inverted first bit b0 to the gates of the first and second transistors TR1, TR2. In certain embodiments, the transmitter circuit 400 may further include another inverter IV3 to receive a second bit b₁ of the electronic data b₀ b₁ for equalizing the loading on the output of the previous stage. Additional logic may be used for providing tri-state-ability for the transmitter circuit 400.

The NAND gate 451 receives the first and second bits b₀, b₁ of the two-bit electronic date b₀ b₁, and performs a NAND operation on the bits b₀ and b₁. The NAND gate 451 provides an output to the gate of the third transistor TR3. The NOR gate 452 receives the first and second bits b₀, b₁ of the two-bit electronic date b₀b₁, and performs a NOR operation on the bits b₀ and b₁. The NOR gate 452 provides an output to the gate of the fourth transistor TR4.

The operation of the transistors TR1-TR4 and the resulting voltage levels at the output 470 depending on the first and second bits b₀, b₁ are shown in Table 2. In Table 2, V_(DSTR5) represents the drain-source voltage of the fifth transistor TR5. V_(DSTR6) represents the drain-source voltage of the sixth transistor TR6. In addition, the four voltage levels generated by the transmitter circuit 400 are illustrated in FIG. 5.

TABLE 2 b₀ b₁ TR1 TR2 TR3 TR4 V_(OUT) 1 1 ON OFF ON OFF V_(DD) 1 0 ON OFF OFF OFF V_(DD) − V_(DSTR5) 0 1 OFF ON OFF OFF V_(DSTR6) 0 0 OFF ON OFF ON 0

In Table 2 and in other embodiments described below, the V_(OUT) levels are those that can be obtained when the transistors (e.g., the transistors TR1-TR4) act as ideal switches. In reality, the transistors may generate small additional voltage drops that produce minor differences in the V_(OUT) levels.

During the operation of the transmitter circuit 400, power is dissipated only during transitions of electronic data. Thus, the power dissipation depends on the frequency of the data transmission. For example, at a data transmission frequency of about 1 Gb/s, the average power dissipated by the circuit of FIG. 4 may be about nine times less than the average power dissipated by the circuit of FIG. 2, which was described as dissipating power based on the signal level for three out of the four possible levels.

Referring to FIG. 6, another embodiment of a transmitter circuit for multi-level signaling will now be described. The illustrated transmitter circuit 600 is configured to generate a four level voltage signal, i.e., a 4-PAM signal, based on two binary digits b0, b1. The transmitter circuit 600 includes a first set of switches 610, a second set of switches 620, a first voltage drop circuit 630, and a second voltage drop circuit 640, an encoder 650, a data input 660 and a signal output 670. The electrical connection among the foregoing components can be described above with respect to the circuit 300A of FIG. 3A. In addition, the configurations of the components are the same as those of the components of the circuit 400 of FIG. 4 except for the first and second voltage drop circuits 630, 640.

In the illustrated embodiment, the first voltage drop circuit 630 includes a diode-connected fifth transistor TR5. The gate of the fifth transistor TR5 is connected to the source/drain of the first transistor TR1. The second voltage drop circuit 640 includes a diode-connected sixth transistor TR6. The gate of the sixth transistor TR6 is connected to the drain/source of the second transistor TR2.

The operation of the transistors TR1-TR4 and the resulting voltage levels at the output 670 depending on the first and second bits b₀, b₁ are shown in Table 3. In Table 3, V_(DSTR5) represents the drain-source voltage of the fifth transistor TR5. V_(DSTR6) represents the drain-source voltage of the sixth transistor TR6.

TABLE 3 b₀ b₁ TR1 TR2 TR3 TR4 V_(OUT) 1 1 ON OFF ON OFF V_(DD) 1 0 ON OFF OFF OFF V_(DD) − V_(DSTR5) 0 1 OFF ON OFF OFF V_(DSTR6) 0 0 OFF ON OFF ON 0

Referring to FIG. 7, another embodiment of a transmitter circuit for multi-level signaling will now be described. The illustrated transmitter circuit 700 is configured to generate a four level voltage signal, i.e., a 4-PAM signal, based on two binary digits b₀, b₁. The transmitter circuit 700 includes a first set of switches 710, a second set of switches 720, a first voltage drop circuit 730, and a second voltage drop circuit 740, an encoder 750, a data input 760 and a signal output 770. The electrical connection among the foregoing components can be described above with respect to the circuit 300A of FIG. 3A. In the illustrated embodiment, the configurations of the components are the same as those of the components of the circuit 400 of FIG. 4 except for the first and second voltage drop circuits 730, 740.

In the illustrated embodiment, each of the first and second voltage drop circuits 730, 740 may include a p-n junction diode. The first voltage drop circuit 730 may include a first diode D1 configured to flow current in a direction from a voltage source V_(DD) to the source/drain of the first transistor TR1. The second voltage drop circuit 740 may include a second diode D2 configured to flow current in a direction from the source/drain of the second transistor TR2 to ground.

The operation of the transistors TR1-TR4 and the resulting voltage levels at the output 770 are shown in Table 4. In Table 4, V_(BID1) represents the built-in potential of the first diode D1, i.e., a potential difference formed across the first diode D1 when an equilibrium condition is reached. V_(BID2) represents the built-in potential of the second diode D2.

TABLE 4 b₀ b₁ TR1 TR2 TR3 TR4 V_(OUT) 1 1 ON OFF ON OFF V_(DD) 1 0 ON OFF OFF OFF V_(DD) − V_(BID1) 0 1 OFF ON OFF OFF V_(BID2) 0 0 OFF ON OFF ON 0

Referring to FIG. 8, yet another embodiment of a transmitter circuit for multi-level signaling will now be described. The illustrated transmitter circuit 800 is configured to generate a four-level voltage signal, i.e., a 4-PAM signal, based on two binary digits b₀, b₁. The transmitter circuit 800 includes a first set of switches 810, a second set of switches 820, a first voltage drop circuit 830, and a second voltage drop circuit 840, an encoder 850, a data input 860 and a signal output 870. The electrical connection among the foregoing components can be described above with respect to the circuit 300A of FIG. 3A. In the illustrated embodiment, the configuration is the same as the circuit 400 of FIG. 4 except for the first and second voltage drop circuits 830, 840.

In the illustrated embodiment, each of the first and second voltage drop circuits 830, 840 includes two serially connected pn-junction diodes. The first voltage drop circuit 830 may include a first diode D1 and a third diode D3 configured to flow current in a direction from the voltage source V_(DD) to the source/drain of the first transistor TR1. The second voltage drop circuit 840 may include a second diode D2 and a fourth diode D4 configured to flow current in a direction from the source/drain of the second transistor TR2 to ground.

The operation of the transistors TR1-TR4 and the resulting voltage levels at the signal output 870 are shown in Table 5. In Table 5, V_(BID1) represents the built-in potential of the first diode D1. V_(BID2) represents the built-in potential of the second diode D2. V_(BID3) represents the built-in potential of the third diode D3. V_(BID4) represents the built-in potential of the fourth diode D4.

TABLE 5 b₀ b₁ TR1 TR2 TR3 TR4 V_(OUT) 1 1 ON OFF ON OFF V_(DD) 1 0 ON OFF OFF OFF V_(DD) − V_(BID1) − V_(BID3) 0 1 OFF ON OFF OFF V_(BID2) + V_(BID4) 0 0 OFF ON OFF ON 0

In other embodiments, each of the first and second voltage drop circuits may have three or more p-n junction diodes connected in series. In certain embodiments, each of the first and second voltage drop circuits may have at least one p-n junction diode and at least one diode-connected transistors connected in series. In this manner, the two middle voltage levels of the four output voltage levels in 4-PAM can be further adjusted.

Referring to FIG. 9, another embodiment of a transmitter circuit for multi-level signaling will now be described. The illustrated transmitter circuit 900 is configured to generate an eight-level voltage signal, i.e., an 8-PAM signal, based on three binary digits b₀, b₁, b₂. The transmitter circuit 900 includes a first to fourth sets of switches 910, 920, 930, 940, a first to sixth voltage drop circuits 915 a, 915 b, 925 a, 925 b, 935 a, 935 b, an encoder 950, a data input 960 and a signal output 970. The configuration of each of the first to fourth sets of switches 910, 920, 930, 940 can be as described above with respect to the first set of switches of FIG. 3.

The first voltage drop circuit 915 a, the first set of switches 910, and the second voltage drop circuit 915 b are connected between a voltage source V_(DD) and ground. The first set of switches 910 may include a first pull-up transistor 910 a and a first pull-down transistor 910 b. A first node N1 between the first pull-up transistor 910 a and the first pull-down transistor 910 b is electrically connected to the signal output 970. The first voltage drop circuit 915 a is connected between the voltage source V_(DD) and the first pull-up transistor 910 a. The second voltage drop circuit 915 b is connected between the first pull-down transistor 910 b and ground.

The third voltage drop circuit 925 a, the second set of switches 920, and the fourth voltage drop circuit 925 b are connected in order between a voltage source V_(DD) and ground. The second set of switches 920 may include a second pull-up transistor 920 a and a second pull-down transistor 920 b. A second node N2 between the second pull-up transistor 920 a and the second pull-down transistor 920 b is electrically connected to the signal output 970. The third voltage drop circuit 925 a is connected between the voltage source V_(DD) and the second pull-up transistor 920 a. The third voltage drop circuit 925 b is connected between the second pull-down transistor 920 b and ground.

The fifth voltage drop circuit 935 a, the third set of switches 930, and the sixth voltage drop circuit 935 b are connected in order between a voltage source V_(DD) and ground. The third set of switches 930 may include a third pull-up transistor 930 a and a third pull-down transistor 930 b. A third node N3 between the third pull-up transistor 930 a and the third pull-down transistor 930 b is electrically connected to the signal output 970. The fifth voltage drop circuit 935 a is connected between the voltage source V_(DD) and the third pull-up transistor 930 a. The sixth voltage drop circuit 935 b is connected between the third pull-down transistor 930 b and ground.

The fourth set of switches 920 is connected between the voltage source V_(DD) and ground. The fourth set of switches 920 may include a fourth pull-up transistor 940 a and a fourth pull-down transistor 940 b. A fourth node N4 between the fourth pull-up transistor 940 a and the fourth pull-down transistor 940 b is electrically connected to the signal output 970.

In the illustrated embodiment, each of the first to sixth voltage drop circuits 915 a, 915 b, 925 a, 925 b, 935 a, 935 b may include one or more diode-connected transistors connected in series. In other embodiments, each of the first to sixth voltage drop circuits 915 a, 915 b, 925 a, 925 b, 935 a, 935 b may include one or more pn-junction diodes connected in series. The sizes of the first to sixth voltage drop circuits 915 a, 915 b, 925 a, 925 b, 935 a, 935 b can be selected so as to provide six middle ones of eight voltage levels, as described below. In certain embodiments, each of the first to sixth voltage drop circuits 915 a, 915 b, 925 a, 925 b, 935 a, 935 b may include one or more diode-connected transistors and one or more pn-junction diodes connected in series.

The encoder 950 receives electronic data from the data input 960, and converts it into control signals S1-S8 for selectively switching on/off components of the first to fourth set of switches 910, 920, 930, 940, as described below in Table 6. In the illustrated embodiment, the encoder 950 generates control signals from three bits b₀, b₁, and b₂ of electronic data.

During operation, the transmitter circuit 900 may generate signals having one of eight voltage levels V1-V8 (V1>V2>V3>V4>V5>V6>V7>V8). In one embodiment, V1-V8 may each represent three-bits (b₀ b₁ b₂): 111, 110, 101, 100, 011, 110, 001 and 000, respectively. The signal levels representing the binary values are summarized in Table 6 below. In table 6, V_(D1), V_(D2), V_(D3), V_(D4), V_(D5), V_(D6) represent the voltage differences across the first to sixth voltage drop circuits 915 a, 915 b, 925 a, 925 b, 935 a, 935 b, respectively. In the illustrated embodiment, the relationships between the voltage differences can be as follows: V_(D1)=V_(D2), V_(D3)=V_(D4), V_(D5)=V_(D6), V_(D1)>V_(D3)>V_(D5). In Table 6, the numbers 910 a, 910 b, 920 a, 920 b, 930 a, 930 b, 940 a, 940 b represent the pull-up and pull down transistors in FIG. 9. A skilled artisan will appreciate that various other configurations of switching circuits can be used for providing multi-level signals in combination with voltage drop circuits, as described above.

TABLE 6 b₀ b₁ b₂ 910a 910b 920a 920b 930a 930b 940a 940b V_(OUT) 1 1 1 OFF OFF OFF OFF OFF OFF ON OFF V_(DD) 1 1 0 OFF OFF OFF OFF ON OFF OFF OFF V_(DD) − V_(D5) 1 0 1 OFF OFF ON OFF OFF OFF OFF OFF V_(DD) − V_(D3) 1 0 0 ON OFF OFF OFF OFF OFF OFF OFF V_(DD) − V_(D1) 0 1 1 OFF ON OFF OFF OFF OFF OFF OFF V_(D2) 0 1 0 OFF OFF OFF ON OFF OFF OFF OFF V_(D4) 0 0 1 OFF OFF OFF OFF OFF ON OFF OFF V_(D6) 0 0 0 OFF OFF OFF OFF OFF OFF OFF ON 0

In certain embodiments, the embodiments described above may be combined with data bus inversion (DBI) schemes. DBI schemes typically favor one binary state over another. A DBI scheme suitable for lowering the number of transitions over time can be combined with any one of the transmitter circuits of the embodiments described above. In one embodiment, as shown in FIG. 10, a processor 1000 may be used to process data digits using such a DBI scheme, and may provide the transmitter circuit with the processed data digits. Other details of FIG. 10 are the same as those of FIG. 3A. This configuration may further lower power consumption while improving signal integrity.

In the embodiments described above, power is dissipated only during the transitioning of electronic data. Thus, power consumption can be effectively reduced.

The transmitter circuits of the embodiments described above can have various applications. For example, the transmitter circuit can be used for short channel applications (e.g., a channel 1010 having a length of about 100 μm to about 10 mm, as shown in FIG. 10), where signal losses and noise are minimal such that a voltage swing can be substantially reduced to, for example, as little as one third of the available rail-to-rail voltage. In one embodiment, an electronic device including multiple integrated circuit (IC) dies stacked over one another may have short channels between the IC dies. The transmitter circuits of the embodiments described above may be formed in one or more of the IC dies for data transmission.

In addition, the transmitter circuits of the embodiments described above can be adapted for various electronic devices. Examples of the electronic devices can include, but are not limited to, consumer electronic products, electronic circuits, electronic circuit components, parts of the consumer electronic products, electronic test equipments, etc. Examples of the electronic devices can also include memory chips, memory modules, circuits of optical networks or other communication networks, and disk driver circuits. The consumer electronic products can include, but are not limited to, a mobile phone, a telephone, a television, a computer monitor, a computer, a hand-held computer, a personal digital assistant (PDA), a microwave, a refrigerator, a stereo system, a cassette recorder or player, a DVD player, a CD player, a VCR, an MP3 player, a radio, a camcorder, a camera, a digital camera, a portable memory chip, a washer, a dryer, a washer/dryer, a copier, a facsimile machine, a scanner, a multi functional peripheral device, a wrist watch, a clock, etc. Further, the electronic device can include unfinished products.

In one embodiment, an apparatus includes a first switch coupled to a first voltage reference and to an output node. The first switch is configured to conduct the first voltage reference to the output node when activated and to be an open circuit when deactivated. The apparatus also includes a second switch coupled to a second voltage reference and to the output node. The second switch is configured to conduct the second voltage reference to the output node when activated and to be an open circuit when deactivated, wherein the second voltage reference has voltage lower than the first voltage reference. The apparatus further includes a first voltage drop circuit coupled to the first voltage reference. The first voltage drop circuit is configured to generate a third voltage reference having voltage lower than the first voltage reference, but higher than the second voltage reference. The apparatus further includes a third switch coupled to the third voltage reference and to the output node. The third switch is configured to conduct the third voltage reference to the output node when activated and to be an open circuit when deactivated. The apparatus further includes a second voltage drop circuit coupled to the second voltage reference. The second voltage drop circuit is configured to generate a fourth voltage reference having a voltage higher than the second voltage reference, but lower than the third voltage reference. The apparatus also includes a fourth switch coupled to the fourth voltage reference and to the output node. The fourth switch is configured to conduct the fourth voltage reference to the output node when activated and to be an open circuit when deactivated. The apparatus also includes an encoder configured to activate at least one of the first switch, the second switch, the third switch, or the fourth switch at least partially in response to input data for the encoder for generation of a multi-level output signal at the output node.

In another embodiment, an apparatus includes a first voltage reference; a second voltage reference lower in voltage than the first voltage reference; and a plurality of voltage drop circuits. Each of the voltage drop circuits is coupled to the first or second voltage reference. Each of the voltage drop circuit is configured to generate a plurality of middle voltage references having voltage levels different from one another. Each of the middle voltage references has voltage lower than the first voltage reference, but higher than the second voltage reference. The apparatus also includes a plurality of switches, each of the switches being coupled to the first voltage reference, the second voltage reference, or one of the middle voltage references and to an output node. Each of the switches is configured to conduct the first voltage reference, the second voltage reference, or one of the middle voltage references to the output node when activated and to be an open circuit when deactivated. The apparatus further includes an encoder configured to activate at least one of the switches at least partially in response to input data for the encoder for generation of a multi-level output signal at the output node.

In yet another embodiment, a method of generating a multi-level output signal for an output node is provided. The method includes: receiving input data; receiving a first voltage reference and a second voltage reference. The second voltage reference is lower in voltage than the first voltage reference. The method also includes generating a third voltage reference, wherein the third voltage reference is lower in voltage than the first voltage reference but higher in voltage than the second voltage reference, and generating a fourth voltage reference, wherein the fourth voltage is lower in voltage than the third voltage reference, but higher in voltage than the second voltage reference. The method further includes coupling the output node to one of the first voltage reference, the second voltage reference, the third voltage reference, or the fourth voltage reference at least partially in response to the received input data for generation of the multi-level output signal for the output node.

Although this invention has been described in terms of certain embodiments, other embodiments that are apparent to those of ordinary skill in the art, including embodiments that do not provide all of the features and advantages set forth herein, are also within the scope of this invention. Moreover, the various embodiments described above can be combined to provide further embodiments. In addition, certain features shown in the context of one embodiment can be incorporated into other embodiments as well. Accordingly, the scope of the present invention is defined only by reference to the appended claims. 

I claim:
 1. An apparatus comprising a transmitter circuit configured to generate a multi-level signal having more than two signal levels, the transmitter circuit comprising: a first voltage drop circuit configured to adjust a first reference voltage by a first difference; a first switch configured to conduct the first reference voltage when activated; and a second switch electrically coupled to the first voltage drop circuit, the second switch configured to conduct the first reference voltage adjusted by the first difference when activated; wherein the transmitter circuit is configured to generate one of the more than two signal levels of the multi-level signal when the first switch is deactivated and the second switch is activated.
 2. The apparatus of claim 1, wherein the transmitter circuit is configured to generate a different one of the more than two signal levels of the multi-level signal when the first switch and the second switch are both activated.
 3. The apparatus of claim 1, wherein the transmitter circuit further comprises: a second voltage drop circuit configured to adjust a second reference voltage by a second difference; a third switch configured to conduct the second reference voltage when activated; and a fourth switch electrically coupled to the second voltage drop circuit, the fourth switch configured to conduct the second reference voltage adjusted by the second difference when activated; wherein the first, second, third, and fourth switches are each electrically coupled to an output node.
 4. The apparatus of claim 3, wherein the first reference voltage is greater than the first reference voltage adjusted by the first difference, wherein the first reference voltage adjusted by the first difference is greater than the second reference voltage adjusted by the second difference, and wherein the second reference voltage adjusted by the second difference is greater than the second voltage.
 5. The apparatus of claim 3, wherein the first reference voltage is provided by a voltage supply, and wherein the second reference voltage is a ground potential.
 6. The apparatus of claim 1, wherein the apparatus comprises multiple integrated circuit dies stacked over one another, wherein the transmitter circuit is formed in one of the multiple integrated circuit dies, and wherein the transmitter circuit is configured to transmit the multi-level signal to another one of the multiple integrated circuit dies over a short channel.
 7. The apparatus of claim 1, wherein the multi-level signal is a 4-PAM signal.
 8. An electronic device comprising: a first integrated circuit die comprising a transmitter circuit, the transmitter circuit comprising a push-pull circuit configured to generate a multi-level signal having at least four signal levels; and a second integrated circuit die stacked adjacent to the first integrated circuit die, the second integrated circuit die configured to receive data from the first integrated circuit over a short channel between the first integrated circuit die and the second integrated circuit die.
 9. The electronic device of claim 8, wherein the short channel has a length between about 100 μm to about 10 mm.
 10. The electronic device of claim 8, wherein the multi-level signal has a voltage swing of about one third of a rail-to-rail voltage.
 11. The electronic device of claim 8, wherein the push-pull circuit comprises a voltage drop circuit configured to adjust a reference voltage; a first switch configured to conduct the reference voltage when activated; and a second switch configured to conduct the adjusted reference voltage when activated, and wherein the push-pull circuit is configured to generate one of the at least four signal levels of the multi-level signal when the first switch is deactivated and the second switch is activated.
 12. The electronic device of claim 11, wherein the push-pull circuit is further configured to generate a different one of the at least four signal levels of the multi-level signal when the first switch and the second switch are both activated.
 13. The electronic device of claim 11, wherein the reference voltage is provided by a voltage source.
 14. The electronic device of claim 8, wherein the push-pull circuit comprises: a first switch configured to couple a first voltage reference to an output node when activated; a second switch configured to couple a second voltage reference to the output node when activated, wherein the second voltage reference is lower in voltage than the first voltage reference; a third switch configured to couple a third voltage reference to the output node when activated, wherein the third voltage reference is lower in voltage than the second voltage reference; and a fourth switch configured to couple a fourth voltage reference to the output node when activated, wherein the fourth voltage reference is lower in voltage than the third voltage reference.
 15. An apparatus comprising: a first switch configured to couple a first voltage reference to an output node when activated; a second switch configured to couple a second voltage reference to the output node when activated, wherein the second voltage reference is lower in voltage than the first voltage reference; a third switch configured to couple a third voltage reference to the output node when activated, wherein the third voltage reference is lower in voltage than the second voltage reference; a fourth switch configured to couple a fourth voltage reference to the output node when activated, wherein the fourth voltage reference is lower in voltage than the third voltage reference; and an encoder configured to selectively activate the first switch, the second switch, the third switch, and the fourth switch based at least partly on input data, wherein the encoder is configured to activate the second switch when the first switch is deactivated.
 16. The apparatus of claim 15, wherein the encoder is configured to activate the third switch when the fourth switch is deactivated.
 17. The apparatus of claim 15, further comprising a voltage drop circuit configured to generate the second voltage reference by decreasing a voltage level of the first voltage reference.
 18. The apparatus of claim 15, further comprising a voltage drop circuit configured to generate the third voltage reference by increasing a voltage level of the fourth voltage reference.
 19. The apparatus of claim 15, wherein the encoder is configured to receive the input data from a processor configured to process data digits using a data bus inversion (DBI) scheme.
 20. The apparatus of claim 15, further comprising a processor configured to process data digits using a data bus inversion (DBI) scheme, wherein the processor is configured to provide the encoder with the processed data digits. 